Silica pigments from fluosilicic acid



Sept. 6, 1966 .L D. NICKERSON ET AL United States Patent O 3,271,107SILICA PIGMENTS FRGM FLUOSILICIC ACID John D. Nickerson and George M.Burkett, Lakeland, Fla., assiguors to International Minerals 8l ChemicalCorporation, a corporation of New York Filed July 31, 1963, Ser. No.293,856 9 Claims. (Ci. 23-182) This invention relates to a novel methodfor the production of silica of high quality from iluosilicic acid. Ingreater particular, the invention relates to a two-stage reaction offluosilicic acid with ammonium hydroxide by which the quality, andparticularly the surface area, of the precipitated silica can be readilycontrolled.

Fluorine-containing gases are produced as a by-product in themanufacture of fertilizer, phosphoric acid, phosphates, and otherphosphorous-containing materials from minerals .such as uorapatite andphosphate rock. When such minerals are reacted with an acid, such asphosphoric acid, sulfuric acid, nitric acid, or hydrochloric acid, whichtreatment is common in preparing useful materials from these minerals,silicon tetrafluoride is liberated. Silicon tetrafl-uoride is -alsoliberated when wet process phosphoric acid produced from phosphate rockor uorapatite is concentrated by evaporation techniques. The liberatedsilicon tetrauoride is usually recovered by absorption in water. Thesilicon tetrafluoride reacts with water to form uosilicic acid (HZSiF).The acid is generally an undesirable by-product having little economicvalue. The iluorine-containing off gases from the manufacture of variousproducts from phosphatic starting materials are recovered usually forthe sole purpose of preventing their escape to .the atmosphere. Theescape of iluorinecontaining oif gases results in serious atmosphericpollution problems, since the gases are both corrosive and toxic. .It istherefore desirable that a worthwhile use be found for these wastematerials.

It is a principal object of this invention to provide an improved methodfor the manufacture of high grade silica from fluosilicic acid.

Another object of this invention is to provide a method for themanufacture of silica from iluosilicic aci-d in which accurate controlof the surface area of the produced silica is obtained.

Yet another object of this invention is to provide a method for themanufacture of silica of improved quality from fluosilicic acid.

Still another object of this invention is to provide a method for theproduction of silica from fluosilicic acid by the reaction of the .acidwith ammonium hydroxide in which dissipation of heat of reaction isconveniently provided.

Yet another object of this invention is to provide a method for themanufacture of `silica from iluosilicic acid in which a high uniformityof product grade can be obtained, and ammonia losses reduced. Otherobjects of the invention will become apparent from the followingspecification.

Briefly, the method of this invention comprises a twostep process forvthe production of silica by the reaction of an aqueous solution offluosilicic acid with ammonium hydroxide in which the fluosilicic acidis rst reacted with a less-than-stoichiometric quantity of ammoniumhydroxide in a reaction zone of high agitation to produce a productslurry including numerous extremely small silica nucleii. The quantitiesof the two reactants is controlled to provide la slurry having a pH inthe range of 6.0 to 8.0. The product slurry is then discharged into areaction vessel in which unreacted uosilicic acid is reacted withammonium hydroxide in amount suflicient to provide a final pH in therange of 8.3 to 9.0. In a preferred embodiment, the

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initial stage of the reaction is carried out in a centrifugal pump.

The reaction is best described with reference to the flow sheet shown inthe drawing. The aqueous solution of iluosilicic lacid which preferablyhas a concentration of about 15 to 25% by weight of lluosilicic acid isflowed from storage vessel 10, through line 12, to centrifugal pump 14.Ammonium hydroxide stored in vessell 16 is lloated through line 18 andadmixed with the aqueous solution of luosilicic acid at the inlet tocentrifugal pump 14. The reaction in the pump is substantiallyinstantaneous and occurs under conditions of very high agitationexisting in the pump. The product slurry is discharged from the pumpthrough line 20 to reaction vessel 22, in which relatively quiescentconditions are maintained. Further neutralization of the slurry iscarried out in batch operation in reaction Vessel 22 by passingadditional ammonium hydroxide from storage vessel 24,v

through line 26, to reaction vessel 22. The second stage of thereaction, which occurs in reaction vessel 22, is permitted to go tosubstantial completion over a time in the range of 20 to 120 minutes.The reaction product is then owed from vessel 22, through line 28, tofilter 30, where silica is removed from the aqueous ammoniumfluoride-containing liquor.

The concentration of the fluosilicic acid employed is not critical, butpreferably will be in the range of about l5 to 25% by weight fluosilicicacid. The concentration of the aqueous ammonium hydroxide reactant isnot critical, and concentrations of about `2.9% have been found to givesatisfactory results. Alternatively, the ammonium hydroxide reactant canbe provided by introducing anhydrous ammonia into the stream of aqueousfluosilicic acid immediately upstream of the inlet to the centrifugalpump. The reaction can be carried out at.

temperatures ranging from 0 C. to ambient temperatures. While it hasbeen found that somewhat superior results are obtainable at reducedtemperatures approaching 0 C., for reasons of simplicity and economicsit is preferred to carry out the reaction at about ambient temperatures.It has been found that excellent temperature control and disposition ofheat of reaction is obtained by carrying out the initial reaction stepin a centrifugal pump. In addition to providing excellent heatdissipation, the pump provides the very high agitation required toproduce numerous small nucleii of silica in the effluent slurry, andfurther removes the reactants rapidly from the reaction zone, which isthe pump itself, and thus further improves the quality of the productslurry. It will be understood, however, that the process can be carriedout by reacting the fluosilicic acid and ammonium hydroxide in anyreaction zone in which a high degree of agitation is provided. Thecentrifugal pump has been found to provide an extremely convenient andelcient reactor.

Pressure conditions within the reactor are not critical, the reactionproceeding satisfactorily at ambient .temperatures or such otherpressures as may naturally occur in the pump. The nal step of thereaction in reaction vessel 22 is likewise carried out at ambienttemperature and pressures, but in batch rather than continuous fashion.The rate of addition of ammonium hydroxide is not critical, but it isdesirable to introduce the total quan-` tity of ammonium hydroxide to bereacted in the reaction vessel over a relatively short period of time,say iive minutes. The separation of the silica from the aqueous motherliquor yat filter 30 is conventional.

The relative quantities of fluosilicic acid and ammonium -hydroxidereacted in the initial step of the reaction in pump 14 has been found toprovide a convenient method for controlling the surface area of theproduct silica. The quantities of uosilicic acid and ammonium hydroxideare controlled to provide a slurry pH in the pump effluent PatentedSept. 6, 1966.

in the range of 6.0 to 8.0. It has been found that the surface area ofthe silica separated at t-he filter approaches about 160 square metersper gram when the pH is controlled to approach 6.0.. The surface area ofthe product during the irst step of the reaction Iixed the temperatureof reaction by the heat capacity of the mixing system. Obviously, theuse of more dilute solutions resulted in somewhat lower reactiontemperatures than the use of silica separated at the lter approachesabout 50 square 5 more concentrated solutions. The level ofprecipitation meters per gram when the pH of the pump eiiiuent apin thepump was in part determined by the quantity of proaches 8.0. In thesecond step of the reaction in reacheat dissipated. tion vessel 22quiescent conditions are maintained and the The viscosities and bulkdensities reported in Table I quantity of ammonium hydroxide added iscontrolled to can be interpreted as a measure of the dispersability ofprovide a final pH in the range of 8.3 to 9.0. Again, the the productsamples. Higher viscosities correlate with deammonium hydroxide reactantcan be added in aqueous creased lbulls densities. Higher surface areasdo not yield solution of any convenient concentration or as anhydroushigher viscosities. This may be due to the partial gel ammonia. The pHvariations within the range of 8.3 to structure of the product and tohigher agglomeration. 9.0 do not have a great effect upon the quality orsurface The data in Table I shows that the bulk densities of the area ofthe product silica. It has been found that redouble precipitates arelower than the bulk densities obcoveries of silica are increased byoperating at higher tained when complete neutralization is provided inthe final pHs and a final pH of about 8.9 is preferred. Variapump. Pumpprecipitation alone produces product silicas tions in the time ofreaction in reaction Vessel 22 Within having Surface areas in excess of150 square meters per the range of to 120 minutes haS been found foaffect gram. Batch precipitation alone produces product silicas. boththe yields and the quality of the Product Silica In 20 having surfaceareas below 50 square meters per grani. general, longer times result inthe Production of more The process of this invention permits themanufacture of hlghly agglomerted SllCaS having Somewhat reduced productsilicas having surface areas in the range -of 50 surface areas. Longerreaction times, however, result in to 150 square meters per gram.Shghtl'y mclfeased slhca Y1e1d5 i The embodiments of the invention inwhich an exclu-` It 1S desirable that he Product S111@ be readlll/ dls25 sive property or privilege is claimed are defined as follows:persabigand that artlde aglomefluon b? rllamtamed 1. In the preparationof silica by the reaction of fluoglllilllflnilnsh ratmfcrglrcfvgaillbeogrge silicio acid with ammonium hydroxide in aqueous medit reucts.It 1s for this reason that 1t 1s preferred to l1m1t the mrtl. th?ltmprovemet gmprhclrnyuigsu the reaction time to a maximum of about 120minutes while 30 ac lon m WP Seara.; s. gesu .th p h d X acceptingslightly reduced silica yields. {a} reflctmg no sl 1cm acl Wlf ammoniumyd rod The process of this invention has been carried out under ide 1n areaction Zone of hlgh agltatlon an, a varying conditions to producesilicas having characteristics JUS1I1gthe proportions 0f the reactantst0 malrliam satisfactory for use as pigments. The conditions and re- 1PH 1rl the range of 6-0 to 8-0, whereby a s111021 sults of theexperiments are set forth in Table I. containing slurry 1s produced;

Table 1 COMPLETE PRECIPITATION IN PUMP (ANHYDROUS AMMONIA) Pump BatchSurface Bulk Viscosity, Percent Test No. Acid Pump Yield Time, Temp.,Batch Area, Density, 5 g. SiO: n

Conc. pH S101, 111111. C. pH mlg. 1b./It.3 100 g.. Wet Cake PercentTurpentlne A 15.75 8.9 97 185 14.5 11A 24, 0o s. s 90 160 11. 23 35 14.0

PARTIAL PRECIPITATION IN PUMP (ANHYD ROUS AMMONIA) 18 7. s2 1s 7.8 187.8 24 7.3 DOUBLE PRECIPITATION (PUMP AND VESSEL) ANHYDROUS AMMONIA 187. s2 63. 7 53 31 s. 9 120 7. 64 60 18. 25 1s 7. 82 63. 7 190 31 s. 9132 6. 96 9o 1s. 55 1s 7. 2 32. 7 9o 33 8. 99 151 8. 09 60 15. 75 15. 6.7 27. 7 se 33 s. s 93 11. 41 29 30. 0 24 7. s 53. 0 77 32 s. 9 116 5. 3560 13. 9 24 7. s 53. o 10o 50 8. s6 93 5. 44 6o 1s. 9 24 7. s 53. 0 12563 s. 9 85 6. 12 65 18. 2

DOUBLE PRECIPITATION (PUMP AND VESSEL) 29% AQUEOUS NHIOH 1s 7. s 57. 335 31 8. 7 147 s. S9 35 16. 8 1s 7. 3 57. 3 100 30 s. 75 139 6. 71 4516. 3 24 7. s (50) 19 47 s. 5 137 s. 62 115 16. 78 24 7. s (5o) 24 47 s.S5 127 s. 36 125 s. 41

In the procedure employed, uosilicic acid in an aqueous (b) thenreacting said slurry in a quiescent reaction medlum having aconcentration as set forth in Table I zone with additional ammoniumhydroxide in an was reacted either with a 29% aqueous solution of am-`amount suicient to provide a iinal pH in the range nonium 'hydroxide orwith anhydrous ammonia, as set 70 of 8.3 to 9.0 for a time of about 20minutes to forth in the table, in a centrifugal pump to the pH indi-:ated in Table I to precipitated silica to a predetermined yield. Theresulting slurry was then passed to a reaction Jessel and reacted underbatch conditions at ambient temaeratures with additional anhydrousammonia. The pump minutes, and separating precipitated silica from theresulting slurry. 2. The method in accordance with claim 1 in which saidaqueous fluosilicic acid has a concentration in the 75 range of 15 to25% by Weight.

3. The method in 4accordance with claim 2 in which the inal pH is about8.9.

4. In the preparation of silica by the reaction of fluosilicic acid withammonium hydroxide in an aqeuous medium, the improvement comprisingcontinuously introducing an aqeuous solution of fluosilicic acid andammonium hydroxide into a `rection zone of high agitation, controllingthe ratio of fluosilicic acid in ammonium hydroxide to maintain in saidreaction zone a pH in the range of 6.0 to 8.0, continuously withdrawingfrom said reaction zone a product slurry, introducing said slurry toreaction vessel, reacting said slurry with additional ammonium hydroxidein an amount suiiicient to provide a final pH of 8.3 to 9.0 for a timeof 20 to l2() minutes, and separating precipitated silica from theslurry.

5. The method in accordance with claim 4 in which said aqueous uosilicicacid has a concentration in the range of 15 to 25% by weight.

6. The method in accordance with claim 4 in which the nal pH is about8.9.

7. In the preparation of silica by the reaction of uosilicic acid withammonium hydroxide in an aqueous medium, the improvement comprisingintroducing an aqueous solution of uosilicic acid to the inlet of acentrifugal pump, reacting said uosilicic acid while in said pump withammonium hydroxide in an amount suiiicient to produce a pump effluenthaving a pH in the range of about 6.0 to 8.0, discharging the pumpeffluent into a reaction vessel, reacting said eluent in said vesselWith ammonium hydroxide in an amount sufficient to provide a a final pHin the range of 8.3 to 9.0 for a time of 20 to 120 minutes, andseparating precipitated silica from the reaction product.

8. The method in accordance with claim 7 in which said aqueous uosilicicacid has a concentration in the range of 15 to 25% by weight.

9. The method in accordance with claim 8 in which the final pH is about8.9.

References Cited by the Examiner UNITED STATES PATENTS 1,235,552 8/1917Chappell 23-88v 2,780,522 2/ 1957 Gloss et a'l. 23-182 XR 3,111,38411/1963 Heckathorn et al. 2 3-182 OSCAR R. VERTIZ, Primary Examiner.

BENJAMIN HENKIN, Examiner.

H. S. MILLER, Assistant Examiner.

1. IN THE PREPARATION OF SILICA BY THE REACTION OF FLUOSILICIC ACID WITHAMMONOUM HYDROXIDE IN AQUEOUS MEDIUM, THE IMPROVEMENT COMPRISINGCARRYING OUT THE REACTION IN TWO SEPARATE STAGES WHICH COMPRISES: (A)REACTING FLUOSILICIC ACID WITH AMMONIUM HYDROXIDE IN A REACTION ZONE OFHIGH AGITATION, AND ADJUSTING THE PROPORTIONS OF THE REACTANTS TOMAINTAIN A PH IN THE RANGE OF 6.0 TO 8.0, WHEREBY A SILICACONTAININGSLURRY IS PRODUCED; (B) THEN REACTING SAID SLURRY IN A QUESCENT REATIONZONE WITH ADDITIONAL AMMONIUM HYDROXIDE IN AN AMOUNT SUFFICIENT TOPROVIDE A FINAL PH IN THE RANGE OF 8.3 TO 9.0 FOR A TIME OF ABOUT 20MINUTES TO 120 MINUTES, AND SEPARATING PRECIPITATED SILICA FROM THERESULTING SLURRY.